1. Field of the Invention
The present invention relates to the application to aqueous calcium carbonate suspensions at high concentration and of pigment size of a compatibility, dispersing, and/or grinding agent, which has the property of hindering the sudden and exaggerated increase of viscosity (shock effect) when an electrolyte in a concentrated form is introduced into the suspension. The invention also relates to the aqueous calcium carbonate pigment suspensions containing the selected compatibility agent, which are used in the field of papermaking and more particularly to coating of the aforesaid aqueous pigment suspensions containing the selected compatibility agent.
2. Discussion of the Background
For coating paper, those skilled in the art have long used pigment suspensions composed chiefly, first, of inorganic pigments such as kaolin, calcium carbonate, talc, titanium oxide, alone or mixed. These pigments are dispersed in an aqueous phase containing at least one binder which is dispersible in water or is water-soluble, and is of natural origin such as starch, casein, carboxymethylcellulose, and/or is a synthetic material such as a styrene-acrylate emulsion or a vinyl copolymer. Finally, the dispersion contains a dispersing agent, for example, a polyphosphate or a water-soluble carboxylic polymer.
The concentration of dry material in these pigment suspensions, particularly of calcium carbonate, has long been close to 70% by weight relative to the total weight.
During this time, since paper-coating techniques have evolved and permit increased coating speeds, it has proved advantageous for the user to utilize pigment suspensions, particularly of calcium carbonate, at higher concentration than in the past, i.e., that now have a concentration of dry substance greater than 70% by weight. Such suspensions with greater contents of inorganic pigments should permit faster coating of the paper and a reduction of the thermal energy necessary for drying, provided that they have the rheological characteristics most favorable for the coating operation, i.e., provided that they are in a state of low viscosity having a high velocity gradient to accomplish much faster coating, or they have a low viscosity at low shear rates to handle them easily, especially in the screening operations preceding the coating operation.
Most pigment suspensions, particularly those of calcium carbonate, when they are intended for coating paper, are made alkaline by introducing an alkali metal hydroxide to bring their pH to a value of at least 9, since it is known that the higher their pH is above 9, the better the stability imparted to them by the binder (the latex) introduced into the suspensions. In order not to risk lowering the high concentration of dry substance currently used for pigment suspensions intended for coating paper the alkalinity of the suspensions is increased more by adding an alkali metal hydroxide in concentrated form to the suspensions. In many cases, the addition of a concentrated hydroxide to suspensions of calcium carbonate with pigment dimensions causes an extremely rapid and large increase in their viscosity, which may go so far as to make the suspensions solid. This phenomenon, known by the expression "shock effect", occurs particularly in calcium carbonate suspensions whenever there is a combination of the conditions of high concentration of dry substance (at least 70% by weight), small particle size of the pigments (at least approximately 40% particles smaller than 1 .mu.m), and addition of an electrolyte in concentrated form.
The absence of at least one of the aforesaid conditions limits, or even better, prevents the shock effect, but involves other disturbances such as intentionally lowering the concentration of dry substance, especially by adding a dilute electrolyte, or maintaining this concentration combined with a voluntarily more coarse pigment particle size. These are all procedures which one of skill in the art attempts to avoid.
Also, to accept the use of aqueous pigment suspensions formulated from calcium carbonate under the conditions just mentioned with reference to concentration and fineness, it would seem desirable to be able to use in the suspensions an agent which simultaneously is able to assist the grinding and dispersion of the pigment particles and to eliminate the shock effect while creating compatibility in situ. This is why the specialized literature for a number of years has reflected research done to try to eliminate this "shock effect" by suggesting dispersing and/or compatibility agents for pigment suspensions.
One such attempt is described in Japanese Patent Application 58-138161, which discloses a dispersing agent for making concentrated aqueous pigment suspensions (about 60% to 70% by weight), of precipitated calcium carbonate have a particle size of from 1 to 3 .mu.m. This dispersing agent is a copolymer containing from about 40 to 99 mole % of a monocarboxylic ethylenic monomer, and from about 0 to 50 mole % of a dicarboxylic ethylenic monomer, and from about 1 to 10 mole % of an ethylenic monomer that has a phosphoric ester group. In this way, aqueous calcium carbonate pigment suspensions have been prepared at a concentration of 70% by weight and with an average particle size between 1 and 3 .mu.m in the presence of the dispersing agent. However, it has not been possible to prevent "the shock effect" when a concentrated electrolyte is added, such as sodium hydroxide (at 50% concentration, for example), since a sudden and exaggerated increase of viscosity has been found.
French Patent Application No. 88 13511 discloses a compatibility agent for interpigment dispersion and grinding for aqueous pigment suspensions formulated from a mixture of inorganic pigments, at least one of which is calcium sulfate hydrate. This agent is a copolymer of the formula: EQU --(A).sub.K --(B).sub.L --(C).sub.M --
in which (A) is an ethylenic monomer with at least one carboxylic group, (B) is an ethylenic monomer having at least one phosphoric or phosphonic group, and (C) is an ethylenic monomer of the ester or substituted type in which L has a value between 5 and 95 parts by weight, with K+L+M=100 parts by weight, and in which K and M separately can have the value 0. However, it has been found that this agent, when used for aqueous pigment suspensions formulated from a mixture of inorganic pigments, at least one of which is calcium sulfate hydrate, with regard to highly concentrated suspensions of calcium carbonate of pigment size, does not permit in every case (but only in some cases) achieving the desired compatibility, i.e., elimination of the shock effect caused by adding an electrolyte concentrated form. A need therefore continues to exist for an improved compatibilizing agent for pigment suspensions.